There are many injectors on the market where the aim is to have high degrees of functionality and automatic features, such as in connection with penetration, injection, setting of dose, priming and covering of the needle after use. At the same time there is a demand on robustness, repeatability and reliability regarding the function, which sometimes might be difficult to meet when dealing with complex multi functions involving many interacting components. When there further are demands on low production costs, especially for devices that are to be used only once, the picture becomes even more complex.
There are in the patent literature numerous solutions to injection devices, the bulk of which never enter the market due to that they do not meet the demands in one way or the other. There is therefore a continuous search for solutions that provide the desired functions that at the same time fulfil the functional and/or economical demands.
Many devices having multi-functions that work in sequence, such as for example penetration, followed by injection, followed by withdrawal, have a subsequent sequence triggered at the end of a previous sequence, for example when the needle has reached full penetration depth, the injection sequence is triggered.
A common design is to have the movement of an action, e.g. an injection stroke, to trigger a subsequent movement. To facilitate this in a robust way, tolerances has to accounted for and in that a trigger point will most often have to be set a portion ahead of the absolute end point. If reaching the end point it may happen that the displacement of a means that is required to trigger the subsequent action may not occur. If, on the other hand the trigger point is set too early in the stroke, the subsequent movement may be triggered so the previous action is not completed within the required time or sequence. In both cases there is a risk is that an intended action may become faulty or not occur at all.
One attempt to solve this is disclosed in document WO 03/097133 discloses a device having a plunger rod arranged in two parts movable to each other. Between the parts a delay mechanism is arranged as a piston/cylinder arrangement with a small bleed hole for evacuating the air when the parts are pressed together, whereby a delay of the movement of the plunger rod during the injection sequence is intended, which should ensure that the medicament container is emptied before the needle is retracted. However, since both penetration and injection are performed by a common spring acting on the plunger rod, the delay mechanism is working during the whole penetration and injection sequence whereby there is a pronounced risk that the delay function has terminated long before the end of the injection sequence is reached. Further, during the injection operation the retraction spring is tensioned.
The document GB 2 414 404 describes an injection device having a reservoir containing highly viscous damping fluid, one strong drive spring for performing both penetration and injection, a weaker spring that is tensioned by the stronger spring during penetration and injection for a needle retraction, and a delay means which is intended to delay the movement of the plunger rod just before the end of an injection, whereby the plunger rod is released from the drive spring and the needle is withdrawn from the injection site. The delay means comprises two decoupling mechanisms wherein the first decoupling is activated just before the stopper within the reservoir reaches its end and the second is activated just before the reservoir is emptied. The decoupling is however delayed due to the time it takes to empty the reservoir, whereby it is ensured that the remaining contents of a container is discharged before the container is released and returned to the original position. However, since the delay starts to act just before the end of the injection there is a pronounced risk that the elastic members of the injection systems, such as rubber compounds or air that by pressure from a plunger rod has become compressed, do not have the enough time to expand to a relaxed state before the retraction is triggered and thereby causing problems with its functionality accuracy and reliability.
The document GB 2 414 404 or GB 2 424 838 or GB 2 396 298 describes an injection device comprising one strong drive spring means for performing both needle penetration and injection by acting on a stopper of a syringe, a weaker spring that is tensioned by the stronger spring during needle penetration and injection for a needle retraction, a delay means having a reservoir containing highly viscous damping fluid, which is intended to delay the movement of the plunger rod just before the end of an injection, whereby the plunger rod is released from the drive spring and the needle is withdrawn from the injection site. The delay means comprises two decoupling mechanisms wherein the first decoupling is activated just before the stopper within the syringe reaches its end and the second decoupling is activated just before the reservoir is emptied. The second decoupling is however delayed due to the time it takes to empty the reservoir, whereby it is ensured that the remaining contents of a container is discharged before the container is released and returned to the original position. However, since the delay starts to act just before the end of the injection there is a pronounced risk that the elastic members of the injection systems, such as rubber compounds or air that by pressure from a plunger rod has become compressed, do not have the enough time to expand to a relaxed state before the retraction is triggered and thereby causing problems with its functionality accuracy and reliability.
There is thus room for further improvements in the technical area of injection devices with a certain amount of automatically and reliable performed functions.